This description relates to power transmission and distribution, and, more particularly, to systems and methods for subsea high-voltage direct current (HVDC) transmission and distribution.
As oil and gas fields in shallow waters diminish, e.g., water depths less than approximately 500 meters (m) (1640 feet (ft.)), producers are tapping offshore fields in deeper waters, e.g., water depths of 500 m (1640 ft.) and greater. Such deep water fields not only include oil and gas production installations that operate far below the surface of the sea, but, also far away from the shore, e.g., greater than approximately 300 kilometers (km) (186 miles (mi)).
In many known subsea oil and gas production systems, typical equipment for such subsea oil and gas recovery and production includes gas compressors and pumps. Electric variable speed drive (VSD) and motor systems are one way to directly power such equipment in deep water environments. Reliable delivery of electric power from a remote utility grid or power generation source facilitates reliable production and processing of oil and gas in subsea locations. Typically, the transmission power requirement may be approximately one hundred megawatts for medium to large oil/gas fields.
As such, some known subsea oil and gas production systems are electric power intensive, and a robust, sturdy, and reliable electrical transmission and distribution (T&D) is required. Therefore, some known subsea oil and gas production systems use alternating current (AC) transmission and distribution systems for delivery of electric power to subsea locations. Such systems typically deliver AC power from a platform or terrestrial location to a subsea transformer through a power cable. Power is transferred from the subsea transformer to subsea AC switchgear through another power cable. The subsea AC switchgear feeds AC power to one or more subsea VSDs through yet another cable, or to other types of electrical loads. The VSDs each provide variable frequency AC power to electric motors through a power cable. Such AC transmission and distribution systems face technical challenges, which become more significant, e.g., when the transmission distance is in excess of one hundred kilometers. For example, the significant reactive power drawn from the distributed subsea cable capacitance restrains the power delivery capability as well as increases the system cost.
Therefore, subsea oil and gas production systems may instead use high-voltage direct current (HVDC) transmission and distribution systems for delivery of electric power to subsea locations. Such HVDC systems typically include a land-based of topside converter substation where the AC-to-DC power conversion is performed. Also, these HVDC T&D systems may include undersea DC-to-AC and DC-to-DC converter stations proximate the subsea oil and gas production systems.
The active subsea power electronics components are generally contained inside enclosures (e.g. pressure vessels) protecting them from the surrounding subsea environment. Further, as pump, motor, and distribution components increase in power and size, the weight increases. Moreover, the electrical connections between components in subsea distribution systems typically require wet-mateable connectors, which are significantly more expensive than dry-mateable connectors. Wet-mateable connectors are used to facilitate making electrical connections between components while underwater, which is in contrast to dry-mateable subsea connectors used to connect electrical components in dry environments, before the connectors are submerged. Furthermore, wet-mateable subsea DC connectors that can withstand high voltage, e. g., 50 kiloVolts (kV) DC or higher, if commercially available, would require a complex and costly design. Moreover, due to the inaccessibility of the components within the enclosures, maintenance on any one component within the enclosure typically requires completely removing the T&D system from service and raising the unitary enclosure from its subsea location onto a ship or other platform.